Xiu Cai repair TV: 29-inch Panasonic color TV TC-29GF10R repair notes - apprentice's care - home appliance repair

Probe current voltage pin 420*4450 head diameter 5.0 over current current and voltage pin

Shunuo varistor / ESD varistor / product complete / Sunlord first-class agent

SMF05C SOT363 ESD electrostatic protection tube 5V

The apprentice was on the phone and said that the Panasonic TC-29GF10R was ruined. Please ask me. Because there is a customer who wants to pick up the goods today, I promised to solve it for him at night.
The apprentice came to my house to wait for me early after dinner. I said don't be nervous. What do you say first?
He said that the power indicator light is on and there is standby voltage, but after pressing the start switch, only the sound of the relay "tick and ticking" is heard, as if the circuit protection is the same, but after the relay is shorted, the switch is pressed, but everything is normal.
I seem to believe that I don't believe it. I came to the computer and opened the circuit diagram. Is the 11V standby voltage normal? The answer is normal. What about 5V? Said that only 4V and more unstable, suspected that there is a problem with the three-terminal regulator IC1214, after replacement, it still does not help, and the relevant parts of the 5V regulator circuit have been checked all over, no abnormalities were found, and even CPu is suspected, but the connection is short-circuited. The relay is all normal, so the problem of CPu is ruled out, but it is impossible to start.
To be honest, he is the most admired among my apprentices. I have a high IQ and I am good at analysis. I am struggling with TV today. It seems to be: "This TV is unusual and dare to play tricks with my apprentice. It’s not a strange fault that is difficult to analyze. How can he be anxious to help me?”
Come to the user's home, put the circuit board in a good position, plug in the power, the indicator light is bright, detect the Q805C pole, and sure enough, there is 11V voltage, press the switch, the relay "tick" sounds, then measure the Q805C pole, only 7V Voltage is unstable, measuring Q805E pole, only 32V voltage, and measuring current limiting resistor R862 rectifier end, 42V voltage is very stable! (The Q805C pole circuit icon is called 53V, and the actual working voltage is 42V). This is my apprentice, has the resistance R862 been checked? He said that the measured resistance is 39 ohms, and the resistance has been peeled off and I don't know how much resistance, so I think it might be good.
Looking at the R862, I have already faded my clothes. I haven’t noticed how much the circuit icon is, but since it is a current limiting resistor, I don’t think it’s big or small. The idea is to press the switch and use the finger tongs to short the R862. Connect, only hear the relay pull-in, TV raster appears! Immediately order the apprentice, remove the R862, put on 4.7 ohms / 1W, resistance, start, hand to the disease!
Xiucai analysis: Because the voltage limiting resistor R862 of the Zener diode Q805 is changed, the output is low and the output is low. The 11V output of the Zener tube is supplied to the IC1214 as a 5V regulator, and also provides the pull-in voltage for the relay. As the R862 becomes larger, the voltage provided is less than the relay pull-in, so the sound of “tick and ticking” appears instead of the circuit protection. At this point, my apprentice has already judged the mistake, and the standby voltage is 11V normal, the resistance R862 Although the inspection is 39 euros, it is considered normal, so it causes more detours and finally jams!
After some resistance values ​​change, although there is still voltage passing, when a large current passes, it tends to heat up to increase the internal resistance, which is the case in this example. Generally, as a current limiting resistor, its resistance is within 10 ohms. The circuit diagram of this machine also does not indicate the specific resistance of R862. However, it should be a fuse resistor from the drawing identification, which is generally between 1 and 2 ohms. The 4.7 Euro I used is large, but it has no effect. This should be used flexibly.
As for the apprentice short-circuit relay, why is everything normal? As seen from the drawing, after the main power supply is rectified, a diode D835 is connected to the output terminal of the auxiliary power supply 11V. Since the main power supply is normal, a stable voltage is provided for the 11V terminal invisibly, so the TV can be started normally. (The following figure)

It is worth noting that some users like to use remote control shutdown for many years, so the standby circuit is always in working state, and the R862 is the worker who is the standby voltage. It is normal for the work to be done all the year round.
thanks for watching! The next article will be again!